The Gaunt Herald was a Zamorakian messenger used by Morytania to give messages to people from Misthalin.
The Herald was originally a messenger who upset a king of Varrock. As punishment, he was handed over to Morytania as a sacrifice to keep them sated. Years later, he was given powers and used to ferry messages from Morytanians to Misthalians. This usually consisted of an offer to turn to Zamorak in exchange for a favour. Legend had it that to refuse the Herald would be to pay a terrible price.
The only people the Herald is known to have appeared before is Lord Ruthven and King Roald. Lord Ruthven was offered a cure for his wife in exchange for his daughter, but he refused, and so his wife died and his daughter was taken and later used as a vampyric creature to help further the plans of the Black Prince.
King Roald was offered the lives of all of Misthalin's citizens if he stepped down and surrendered his own life in favour of Tenebra's child who was the true heir to the Misthalian throne. King Roald furiously refused, and the Herald tried to kill Roald, but Sir Theodore saved him and a guard was hit instead, erupting in flames and his flesh melting. Gideon Gleeman and Layte Aubury killed the Herald with potent fire magic before he could try again.